Television receiver



Nov. 12, 1940. G M0l Jl\|T l0Y1 2,221,104

' TELEVISION RECEIVER Filed Oct. '7, 1958 .ffy o/rpur M2 owPur' A TTORN E Y.

Patented Nov. 12, 1940 UNTE - anni raras P'raN'r orion y* TELEVISION RECEIVER Garrard Mountjoy, Manhasset, N. Yi, assignorto Radio Corporation of America, a corporation of Delaware l Application Qctober 7, 1938, Seral'No. 233,711l

Claims.

nels, one of the carrier frequencies being modulated at video or picture frequency whereas the other carrier is modulated at sound or audio frequency. When these two transmission frequencies are received, a superheterodyne receiving system is conventionally used in which a single oscillator is provided which, when beat against the two received carrier frequencies produces two independent intermediate frequencies,L

one of which is known as the picture intermediate frequency and the other of which is known as the sound intermediate frequency. Inasmuch as a single oscillator is used, the output of the convert-,er or mixer tube, in which the frequency generated by the oscillator and the carrier frequencies are beat or combined, supplies both intermediate frequencies, which, of course, are not identical in frequency and heretofore a common tuned circuit has-not been available which will resonate or peak at both intermediate frequencies. Because of this ordinarily a compromise was reached and the tuning of the intermediate frequency transformers which were to carry both intermediate frequencies were made sufficiently broad that both intermediate frequencies may pass therethrough. The two intermediate frequencies are normally separated soon after being produced in order that each may follow its own amplication channel for subsequently modulating the control electrode of a cathode ray tube to produce a television image and to control the currents applied tothe voice coil of a speaker in order to produce sound waves.

It is therefore one purpose of the present invention to provide a new and improved circuit for the reception of two high frequency carrier waves, each of which is separately modulated, the system being supplied with a single oscillator for producing two separate intermediate frequencies, the circuit being so designed that that portion of the circuitl wherein both intermediate frequencies occur is so designed as to resonate `for both intermediate frequencies.

Another purpose of the present invention resides inthe provision of means whereby a ysingle intermediate frequency'transforiner may be so (ci. iis- 5.3)

affected as to readily admit the transmission of two separate frequencies and to attenuate orreject all other frequencies.

A still further purpose of the present invention resides in the provision of an intermediate frequency transformer and associated elements such 'that the transformer will have the characteristics of peaking or offering low impedance l to two separate intermediate frequencies.

Still another purposejof the present. invention resides in the provision of means which, when used with an intermediate frequency transformer such as that referred to above will forma convenient means whereby the two intermediate frequencies may be Aseparated for individual and separate subsequent amplification.

Variousr other purposes and advantages of the present invention willbecome more apparent tothose skilled in the art from al reading of the following specification and claims, particularly when considered in connection with the drawing, wherein:

The single gure representl a preferred form of the present invention.

VReferring; now to the drawing, an aerial of appropriate design for receiving the transmitted carrier frequencies is provided, which is connected to the primary coil I0 of the antenna transformer.

The-secondary l2 of this transformer is tuned by means of the variable condenser lfl which is connected in vparallel with the secondary coil, in order that the desired carriers may be received. The coil l2 is coupled to the coil l5 and in parallelrvvith which is connected a resistance i8 in order that l.the frequency band response of the receiver will be sufciently broad to include two carrier frequencies. The coil i6 then supplies radio frequency energy tothe control electrode of the converter tube 2t.

An oscillator tube 22 is provided which includes a cathode, a control electrode and an anode. Associated with the oscillator is a coil 2li in parallel with which is connected a variable condenser. 25 forv controlling the frequency of oscillation supplied bythe oscillator. Preferably the condenser 253 and the condenser I4 are connectedtogether so that, as thecondenser itis varied to receive different transmitted frequencies, thev oscillator irequencyV will also bey varied 4in order that the produced intermediate frequencies will remainconstant in. a manner well known in the art of receiving radio signals. The coil 24 and the coil l5 areinductively coupled together, Aso that the oscillations 'of the frequencies supplied lby the oscillator Will alsov be applied-to the control electrode of the converter tube 2l). Inasmuch as the received frequencies and the locally generated frequency are both applied to the control electrode of the converter tube 2li, this tube will have present in its output circuit intermediate frequencies of values depending upon the received frequencies and the oscillator frequency. Inasmuch as a more or less conventional oscillator is shown and inasmuch as the manner in which the received frequencies and the oscillator frequency are combined does not form a part of the present invention, further discussion of the operation of these elements is believed unnecessary, particularly in View of the Vfact that their manner of operation is well known in the art.

Connected to the anode of .the converter tube v20 is a coil 28 and appropriate anode potential is applied to the anode .of the converter tube from a source (not shown) which may be connected to the terminal 30, by way of a decoupling resistance 32 and the coil 28. Connected at the point between the decoupling resistance 32 and the coil 28 is a by-pass condenser 34 which is of such magnitude as to permit intermediate frequencies to be by-passed to ground, to which the cathode of the converter tube 20 is connected by way of the cathode resistor 38. The winding 28 is preferably provided with a magnetic core, the position of which is variable within the coil, whereby the inductance of the coil may be varied in accordance with the position .of the magnetic core. The inductance of the coil is so adjusted to form a parallel resonant circuit at a particular frequency with the distributed capacity of the coil as well as the inter-electrode capacity of the converter tube 20. If such capacity is insufficient for proper tuning a small condenser 29 may be added in parallel with the coil 28. Because of this resonant condition the potential of the anode of the converter tube 20 will fluctuate and these voltage variations are transferred to the control electrode of intermediate amplifier tube 46 by means of the coupling condenser 42. `In order that the input of the intermediate frequency amplifier tube 40 may be tuned, a coil 44 is provided which, like coil 28, is preferably magnetically tuned and its inductance is so adjusted with respect to its distributed capacity as well as the inter-electrode capacity of the tube 4Q, that the coil will resonate at a particular frequency. Here again an additional small condenser 45 may be placed in parallel with the coil 44 if the distributed and inter-electrode capacities are insuicient to produce resonance. Connected in parallel with the coil 44 is a resistance 46 which functions as a damping resistance in order that the band width of the resonant frequency of the transformer may be increased. The one end of the coil 44 is connected to the control electrode of the intermediate frequency amplier tube 40, whereas the other end of the coil 44 is connected through resistance 48 to a variable contact along a potentiometer 5D, one end of which may be supplied with negative p0- tential. By varying the negative bias applied to the control electrode of the intermediate frequency amplifier tube 40, (and also to the control electrode of the converter tube 20) the gain of the system may be controlled. The last mentioned end of the coil 44 is by-passed to ground by way of the condenser 52.

With the invention as so far described, if a single carrier frequency isl received at the aerial, and a single intermediate frequency is produced in the converter tube 20 as a result of beating the carrier frequency and the oscillator frequency as produced by the tube 22, this intermediate frequency will appear in the intermediate frequency transformer and naturally, the lintermediate frequency transformer will be tunedto resonate at this particular intermediate frequency. This tuning, as stated above, may be accomplished by varying the inductance of the coils 28 and 44 so that signal voltage variations of the particular intermediate frequency will be readily transferred to the control electrode of the intermediate frequency amplifier tube 4U. After being amplified by this tube, the signals are again applied to an intermediate frequency transformer and as many stages of intermediate frequency amplification as desired may be used before detection or rectication is employed.

Such a system is more or less conventional, where superheterodyne receivers are employed, but where two carrier frequencies are to be received, and two intermediate frequencies are to be produced as a result of the use of a single oscillator, the system as so far described is not adequate since the intermediate frequency transformers cannot be made to resonate at two intermediate frequencies. In the transmission of television signals, it is of course highly desirable to transmit both the picture signals and the sound signals in order that proper correlation of sight and sound may be had, and the picture and sound signals are ordinarily used to modulate two separate carrier frequencies which normally occupy adjacent transmission bands.

By the provision of certain additional elements in the circuit as above described, the intermediate frequency transformer which is positioned .between the converter tube ZDand the intermediate frequency amplifier tube 40 may be made to resonate at two different intermediate frequencies. In vorder that this may be accompllshed, a condenser 54 and an inductance 56 are connected between the control electrode of the intermediate frequency amplifier tube 40 and ground. Assuming that the intermediate frequency transformer, as explained above, is adjusted to resonate at picture intermediate frequency, then the condenser 54 and the coil 56 are so chosen in value as to resonate at sound intermediate frequency. When this is done, any sound intermediate frequencies which occur at the input of the intermediate frequency amplifier tube 4&3 are lay-passed to ground and are prevented from affecting tube 40 since the condenser 54 and the coil 56 constitute a series resonant circuit which, as stated above, resonate at sound intermediate frequency. At this intermediate frequency then, the' presence of the condenser 54 and the coil-56 effectively connects the control electrode of the amplifier tube 40 to ground. This furthermore amounts to a shorting of the damping resistance 46 and accordingly a shorting of the coil 44 only insofar as sound intermediate frequencies are concerned. This, then, effectively places the condenser 42 in parallel with the winding 28, since the lower end 0f each of the coils 28 and 44 are connected to ground by way of by-pass condenser 34 and 52 respectively. It may be seen, therefore, that the presence of sound intermediate frequencies cause, by reason of the condenser 54 and the inductance 55, an effect similar to placing the condenser 42 in parallel with coil 28 which, of course, causes this portion of the intermediate frequency trans- Vformer to resonate at alower frequency.

intermediate frequency, signal variations of this intermediate frequency may be readily applied to the control electrode of the sound intermediate frequency amplifier tube 60 through the coupling condenser 62, which, of course, is connected between the anode of the converter tube 20 and the control electrode of the sound intermediate frequency amplifier tube 60. The control electrode of this tube is, of course, connected to a source of negative potential through a control grid resistor 64. The .output of the sound intermediate frequency amplier tube 60 may then be applied to an intermediate frequency transformer which is so chosen as to resonate at sound intermediate frequency only, so that it,- as well as anyy desirable subsequent stages of amplification, may be made readily acceptable to' the sound intermediate frequency.

Although it mightappear that this effect would be somewhat deleterious insofar as `the picture intermediate frequency is concerned to which the coil 28 is tuned excluding the effect of the condenser 42, this is not true since insofar as picture intermediate frequencies are concerned the presence of the condenser 54 and the inductance 55 are substantially ineffective and the intermediate frequency transformer as a whole will readily admit picture intermediate frequencies in a manner as tho-ugh the condenser 54 and the inductance 58 were omitted.

Connected in parallel with the inductance 55 is a variable condenser 65 Which, for the purpose 40 of explanation, may be omitted from the circuit but which in practice may be used to vary the effect of the series resonant circuit including the condenser 54 and the inductance 55.

It may be seen from the above, therefore, that by reason of the circuit described the intermediate frequency transformer for coupling the converter tube 2l! and the tube 48 may be made to resonate at two different intermediate frequencies.

For the purpose of more fully explaining the present inventio-n, a set of values will be assumed which, it is to. be understood, are merely theoretical and are not to be taken as the only lvalues at which the system may be made to operate.

It will be assumed that the carrier frequency which is transmitted at the transmitting station and which is modulated at picture or video frequency is 45.25 megacycles. It will be further assumed that the sound carrier frequency which is modulated at sound or audio frequency is of the order of 49.75 megacycles. It will also be assumed that the oscillator which includes the discharge tube 25 and its associated elements supplies a frequency of 58.00 megacycles. When these Values lare assumed, it may be readily seen that the picture intermediate frequency will be 12.75 drnegacycles and the sound intermediate frequency will be 8.25 megacycles. With this set of circumstances, the carrier frequencies, as well as the intermediate frequencies, are spaced an amount equivalent to 4.5 megacycles. The inductance of coil 28 is then so adjusted with respect to the distributed capacity of the coil to- --gether with-the interelectrode capacity iof the inegacycles.

vthe inductance of the coil 44 is so chosen that the coil and its distributed capacity as Well as the capacity of the succeeding tube (and the condenserl 45, if necessary) will also resonate at picture intermediate frequency or 12.75 megacycles. When this is done it is perfectly obvious that picture intermediate frequencies will be readily admitted to the intermediate frequency transformer land will accordingly be applied to the picture intermediate frequency -amplifier tube 40 by way of the coupling condenser 42 and such amplified picture intermediate frequencies will appear at the output of this tube, which, in the drawing carries the designation IFi.

The value of the capacity 54 and the inductance 55 are so chosen as to form a series resonant circuit for the sound intermediate frequencies which, as stated above, is 8.25 megacycles. The series resonant circuit including the condenser 54 and the inductance 55, as stated above, effectively represent a substantial short circuit or very low resistance insofar as sound'intermediate frequencies are concerned. This effect amounts to the application of the condenser 42 across the coil 28 and when thecapacity element of a parallel resonant circuit is increased the frequency at which the circuit will resonate is decreased. The value of the condenser 62 is so adjusted that the coil 28, together with its distributed capacity, the inter-electrode capacity of the converter tube 2B and the capacity of the condenser 42 (as well as condenser 29, if used) forma circuit which is resonant at sound intermediate frequency or 8.25 v

This sound intermediate frequency is then applied to the control electrode of the sound intermediate frequency amplifier tube 50, the output of which is connected to succeeding stages of sound intermediate frequency ampliflcation, the number of which may be chosen in accordance with the desired necessities. The intermediate frequency transformers which are connected to the output of the amplier tube are, of course, made to resonate at sound intermediate frequency only. The output from amplifier tube 50 is represented on the drawing as IFz.

Inasmuch as subsequent intermediate frequency stages of amplification for either the picture or sound channel may be of the type conventionally used, these succeeding stages are not shown in the drawing since they form no part of the present invention. After such intermediate frequency amplification appropriate detection and subsequent video and audiofrequency amplification is employed.

Although the present invention is described as being particularly adapted to the reception of television signals wherein both video' frequencies and sound frequencies are transmitted, it is: to be understood that the present invention may be readily applied to any system wherein two relatively adjacent frequencies are to be received and wherein two intermediate frequencies are developed. lIt is no-t necessary that the two frequencies be specifically videogand audio frequencies since it is entirely immaterial as to the source of the signals which are used to modulate the two carriers. In fact, the signals for modulating' the two carrier frequencies may be derived fro-m any two desirable variabile frequency signal sources. Furthermore, it is obvious thatthe invention could be applied to a binaural system in which sounds are picked up on two spaced microphones,

lio

the outputs from which could be used to modulate two carrier frequencies.` Speakers at the receiving station, spaced in accordance with the spaced microphones, would then produce a binaural effect.

Furthermore, even though magnetic tuning of the intermediate frequency transformers is shown in the drawing and described in the above speciiication, it is to be understood that intermediate frequency transformers having fixed inductances and variable capacities (capacity tuning) could well be used particularly where carrier frequencies of lower frequencies are used. The use of magnetic tuning is, however, particularly desirable when carrier frequencies of the order of 50 megacycles are used, and for this reason such intermediate frequency transformers are shown.

It is not necessary, of course, that the size of the condenser 42 be so chosen as to cause the coil 28 to exactly resonate at the lower of the two intermediate frequencies since an approximation of resonation materially improves the operation of the system. In television receivers the value of the condenser if affects the Width of the picture intermediate frequency band and the condenser is generally chosen of such a value that the desired band of frequencies will be passed. This Value may not cause exact resonance of coil 28 at sound intermediate frequency but possibly only an approximation. lThe size of the coil 28 may, however, be so chosen that both situations will be satisfied.

Various other modifications and alterations may be made in the present invention without departing from the spirit and scope thereof, and it is to be understood that these modifications are to be considered within the purview of the present invention, except as limited by the hereinafter appended claims.

I claim:

l. In a radio receiving circuit, means for receiving two carrier frequencies, means including an oscillator for producing a first and a second intermediate frequency, an intermediate frequency transformer including an input coil and an output coil, said coils being tuned to resonance at one of the intermediate frequencies, a coupling condenser connected to one end of each of the coils, means for coupling the other end of each of the coils, and means including a serially connected condenser and an inductance connected in parallel with said output coil and tuned to resonate at `the other of the intermediate frequency whereby when said other intermediate frequency is present, said coupling condenser will be effectively connected in parallel with said input coil, the value of said coupling condenser being such that said input coil together with said coupling condenser will be made to resonate at said other intermediate frequency.

2. A radio receiving system including means for receiving two carrier frequencies, a local oscillator, means for beating the local oscillations against the received carrier frequencies to produce a rst and a second intermediate frequency, an intermediate frequency transformer including a parallel resonant input circuit and a parallel resonant output circuit, said circuits being tuned to resonate at said first intermediate frequency, means including a coupling condenser for effectively connecting said resonant circuits, and a series connected condenser and inductance tuned to resonate at said second intermediate frequency connected in parallel with said output resonant circuit whereby when said second intermediate frequency is present said coupling condenser will be effectively placed in parallel with said input resonant circuit, the value of said condenser being so chosen that said input resonant circuit together with said coupling condenser will then be resonant at said second intermediate frequency.

3. A radio receiver including means for receiving two modulated carrier frequencies, an oscillation generator, means including an electron discharge device for combining said received oscillations and said locally generated oscillations to produce a first and a second intermediate frequency, a parallel resonant circuit connected to the output of said electron discharge device, said circuit being adjusted to resonate at said first intermediate frequency and constituting the input side of an intermediate frequency transformer, a second parallel resonant circuit, means for connecting said second parallel resonant circuit to an electron discharge amplifying device, means including a condenser for coupling said resonant circuits whereby energy of the said first intermediate frequency will be supplied to said amplifying device, means including a condenser and an inductance connected in parallel with said second resonant circuit, said condenser and inductance being tuned in series resonance to said second intermediate frequency whereby when said second intermediate frequency is present, said series resonant circuit will in effect short circuit said second resonant circuit to in effect place said coupling condenser in parallel with said rst resonant circuit, the value of said condenser being so chosen that said first resonant circuit and the effectively parallel connected coupling condenser will be made resonant at said second intermediate frequency.

fi. A radio receiver including means for receiving two modulated carrier frequencies, an oscillation generator, means including an electron discharge device for combining said received oscillations and said locally generated oscillations to produce a iirst and a second intermediate frequency, a parallel resonant circuit connected to the output of said electron discharge device, said circuit being adjusted to resonate at said first intermediate frequency, a second parallel resonant circuit, means for connecting said second parallel resonant circuit to an electron discharge amplifying device, means including a condenser for coupling said resonant circuits whereby energy of the said first intermediate frequency will be supplied to said amplifying device, means including a series resonant circuit connected in parallel with said second resonant circuit, said series resonant circuit being tuned to resonance atk approximately said second intermediate frequency whereby when said second intermediate frequency is present, said series resonant circuit will in effect short circuit said second resonant circuit to in effect place said coupling condenser in parallel with said iirst resonant circuit, the value of said condenser being so chosen that said first resonant circuit and said coupling condenser will be made resonant at approximately said second intermediate frequency, and means for applying said intermediate frequencies to separate amplifying channels.

5. A television receiving circuit for simultaneously receiving a video modulated carrier frequency and an audio modulated carrier frequency comprising an oscillator, means including said oscillator for producing a picture intermediate frequency and a sound intermediate frequency of a frequency less than the picture intermediate frequency, an intermediate frequency transformer including input and output parallel resonant circuits tuned to picture intermediate frequency, means including a coupling condenser for coupling said resonant circuits, and a series resonant circuit tuned to sound intermediate frequency connected across said output resonant circuit whereby at sound intermediate frequency said coupling condenser will be effectively connected across said input resonant circuit to cause said input resonant circuit together with said coupling condenser to resonate substantially at sound intermediate frequency.

6. A system for separating two frequencies one from the other comprising a pair of parallel resonant circuits adjusted to resonate at one of the frequencies, means for impressing both frequencies across one of the resonant circuits, means including a capacity for coupling the resonant circuits, and a series resonant circuit adjusted to resonate at the' other of the frequencies connected across the other of the resonant circuits whereby said capacity is effectively placed in parallel with said one resonant circuit, the value of the capacity means being so chosen that said one resonant circuit together with said capacity will also be resonant at the said other frequency.

'7. A system 'for separating two frequencies comprising a pair of resonant circuits adjusted to o resonate at one of the frequencies, means for impressing both frequencies across one of the resonant circuits, means including a condenser for coupling the resonant circuits whereby said one frequency will be readily available across the other resonant circuit, and a series resonant circuit adjusted to resonate at the other of the frequencies and connected across the said other of -the resonant circuits whereby said condenser is effectively placed in parallel with said one resonant circuit, the value of the condenser being so chosen that said one resonant circuit and the effectively parallel connected condenser will be substantially resonant at the said other frequency, in order that said other frequency may be readily available across said one resonant circuit.

8. A television receiving circuit for simultaneously receiving a video modulated carrier frequency and an audio modulated carrier frequency comprising a single oscillator, means including said oscillator for producing a picture intermediate frequency and a sound intermediate frequency, the frequency of the latter being less than the frequency of the former, an intermediate frequency transformer including input and output parallel resonant circuits, means including a coupling condenser for coupling said resonant circuits, and a series resonant circuit connected across said output resonant circuit,'said parallel resonant circuits being adjusted to resonate at intermediate frequency said coupling condenser will be effectively connected across said input resonant circuit to cause said input resonant ciry cuit together with said coupling condenser to simultaneously resonate substantially at soun intermediate frequency.

'9. A system for'separating electrical energies of two frequencies one from the other comprising a first and second resonant circuit, each adjusted normally to resonate at one of the frequencies, a connection from each circuit to a point of fixed potential, means for impressing both frequencies across the first of the resonant circuits, capacity means `for transferring the energy from the first to the second of the resonant circuits, and a series resonant circuit adjusted to resonate at the second of the two frequencies connected in parallel with the second of the parallel resonant circuits and also connected to said point of fixed potential whereby said capacity means is effectively placed in parallel with the first said resonant circuit withy respect to the resonant frequency of the series circuit, said capacity means being of Such value that the first of said resonant circuits together with said capacity means will be resonant at `the said other frequency.

10. A system for separating two frequencies comprising a pair of resonant circuits adjusted to resonate at one ofthe frequencies, means for impressing both frequencies across one of the resonant circuits, means including a condenser for coupling the resonant circuits whereby said one frequency will be readily available across the other resonant circuit, a series resonant circuit tion of the series :resonant circuit to vary the effect of said circuit.

GARRARD MouNfiuoY.` 

